Antiserum produced by immunizing rabbits with embryonic chicken spleen cells showed, after appropriate absorption with cell suspensions, specificity for fetal spleen cells in membrane immunofluoresoence tests. However, when the original antiserum was absorbed with tissue homogenates and tested on cryostat tissue sections, it reacted specifically with the reticular framework which separates the cortical and medullary lymphocytes in the follicles of the adult bursa of Fabricius. It is suggested that the reticulin may influence bursal lymphoid differentiation.